console.log(typeof(a)) // undefined
console.log(typeof(undefined)) // undefined
console.log(typeof(NaN)) // number
console.log(typeof(null)) // object

var a = "123abc"

console.log(typeof(a)) // string
console.log(typeof(+a)) // number
console.log(typeof(!!a)) // boolean
console.log(typeof(a + "")) // string

console.log(1 == "1") // true
console.log(NaN == NaN) // false
console.log(NaN == undefined) // false
console.log(parseInt(a)) // 123
console.log(typeof(typeof(a))) // string
